1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus, particularly to a toner supply control in a developing device using a one-component developer (toner).
2. Description of the Related Art
In recent years, due to the growing awareness of environmental protection, there has been an increasing need for developing devices capable of achieving a longer life through the replenishment of toner even for those using a one-component developer, which have been conventionally disposable. In this case, a method for detecting a toner remaining amount of the developing device by means of a remaining amount detecting sensor and replenishing the toner from a toner container connected with the developing device if the toner remaining amount decreases is generally used as a method for replenishing the toner.
On the other hand, image forming apparatuses are being entirely downsized due to the recent years' space saving demand and, accordingly, developing devices are also being downsized. If toner is replenished by detecting a toner remaining amount in a downsized image forming apparatus, the toner is replenished after the toner remaining amount is decreased to an extremely low level.
In this case, the replenished toner might transfer straight to a developing roller to form linear nonuniformity. If the replenished toner concentrates near the developing roller, the toner remaining in the developing device thus far might cause a charge failure, thereby causing an image defect (e.g. fogging).
Thus, in the case of replenishing the toner into the downsized developing device, it is effective to conduct such a management that the toner is replenished while a sufficient amount of the toner still remains.
The arrangement of a full detecting sensor in an upper part of a toner hopper and the arrangement of a remaining amount detecting sensor below the full detecting sensor (see, for example, Japanese Unexamined Patent Publication No. 2003-223045) is known as prior art for conducting this type of management. In this prior art, the toner supply is stopped if the full state of the toner is detected by the full detecting sensor at the time of filling the toner, and the replenishment of the toner is started when the remaining amount detecting sensor detects that the toner remaining amount has decreased.
However, the above prior art is studied for two-component development using a two-component developer and cannot be applied as it is to one-component development using a one-component developer. In other words, according to the prior art, the remaining amount detecting sensor detects that the toner remaining amount has decreased and the full state is detected using the other sensor at the time of replenishment.
If such a method is applied to one-component development, the toner replenishment is, due to the space restriction, started after the toner remaining amount decreases to an extremely low level. Thus, a large amount of toner is replenished, which might cause the charge failure and the linear nonuniformity.
Accordingly, in the case of the one-component development, it is effective to arrange one toner amount sensor near a full position of the developing device and to replenish the toner little by little while the full state is monitored by the toner amount sensor. In this way, occurrences of the charge failure and the linear nonuniformity can be prevented for a certain period.
However, in the case of stopping the disposable use of the developing device using the one-component developer and aiming to further extend the life as described above, the toner in the developing device is deteriorated by the long-term use. In this case, the charge failure of the toner eventually occurs even if the toner is replenished little by little.